An amorphous oxide semiconductor thin film has a high carrier mobility, i.e., a high electron mobility, and a large optical bandgap compared with amorphous silicon (a-Si) that has been used for a thin film transistor, and can be formed at low temperature. The amorphous oxide semiconductor thin film is therefore expected to be applied to a next-generation display requiring large size, high resolution, and high-speed drive, and to a transparent display or a flexible display manufactured on a resin substrate having low heat resistance.
Among such oxide semiconductor thin films, an amorphous oxide semiconductor thin film including indium (In), gallium (Ga), zinc (Zn), and oxygen (O) (a-In—Ga—Zn—O, hereinafter, also referred to as “a-IGZO” or simply “IGZO”) is particularly preferred to be used because of its high carrier mobility. For example, NPTL 1 discloses a TFT in which an oxide semiconductor thin film of In:Ga:Zn=1.1=1.1=0.9 (atomic percentage) is used as a semiconductor layer (active layer) of TFT. PTL 1 discloses an amorphous oxide semiconductor including elements such as In, Zn, Sn, and Ga, and Mo, in which a ratio of an atomic number of Mo to an atomic number of all metals in the amorphous oxide semiconductor is 0.1 to 5 at %, and discloses TFT using an active layer including IGZO containing Mo in Example.
It is however known that properties of the oxide semiconductor thin film are varied due to a film formation step and subsequent heat treatment. For example, carrier concentration that dominates TFT characteristics greatly varies due to defect levels caused by lattice defects formed during the film formation step and impurities such as hydrogen in the film. This allows the TFT characteristics to be easily varied. In a manufacturing process of a display or the like, therefore, the following is important in light of improving productivity: Properties of an oxide semiconductor thin film are evaluated, and results of the evaluation are fed back to adjust a manufacturing condition for quality control of film quality.
With the TFT using the oxide semiconductor thin film, it is reported that TFT characteristics greatly vary depending on a process condition performed in a step other than the film formation step of the oxide semiconductor and the subsequent heat treatment. For example, NPTL 2 discloses that when an oxide semiconductor thin film is annealed, an electronic state in the oxide semiconductor film varies depending on a type of a gate insulating film used in the TFT, which resultantly greatly affects the TFT characteristics. NPTL 3 reports in detail that TFT characteristics are greatly affected by a type of a protective film formed on a surface of an oxide semiconductor thin film.
The TFT using the oxide semiconductor thin film requires not only mobility as a basic transistor characteristic but also good stress resistance. The stress resistance means that even if a semiconductor element such as a transistor receives stress, for example, continuous light irradiation or continuous application of a gate voltage, the semiconductor element maintains good characteristics without any change.
In one requirement for stress resistance, threshold voltage (hereinafter, also referred to as “Vth”) does not shift in drain current-gate voltage characteristics (hereinafter, also referred to as “I-V characteristics”), which means a small amount of change in Vth (hereinafter, also referred to as “ΔVth”) between before and after stress application. For example, in an organic EL display, a positive voltage (hereinafter, also referred to as “positive bias”) is continuously applied to a gate electrode of a drive TFT during light emission of the organic EL display, which disadvantageously varies a switching characteristic of the organic EL display.
In addition, a good initial repetition characteristic is necessary as the stress resistance. The initial repetition characteristic means a difference between Vth calculated from I-V characteristics obtained at first measurement and Vth calculated from I-V characteristics obtained after multiple times of measurement when the I-V characteristics are measured multiple times after TFT is manufactured. The smaller the difference (hereinafter, also referred to as “shift of Vth”), the better.
With the stress resistance, Vth of TFT is also necessary to be controlled within a proper range. If Vth has a minus value, current flows when the gate voltage is not applied, leading to an increase in power consumption. On the other hand, if Vth has an extremely large positive value, TFT operation requires a large voltage to be applied to a gate.
If such a switching characteristic is thus varied due to stress by voltage application during use of TFT, reliability of the display itself, such as a liquid crystal display or an organic EL display, is reduced. It is therefore desired to improve stress resistance particularly after application of a positive bias.
In typical evaluation of stress resistance, there has been a problem that measurement under a longtime stress condition must be actually performed after TFT is manufactured through formation of a gate insulating film and a passivation insulating film on an oxide semiconductor thin film, and provision of electrodes.